Direct measurement of the mass difference of $^{72}$As-$^{72}$Ge rules out $^{72}$As as a promising $\beta$-decay candidate to determine the neutrino mass

TL;DR

This study precisely measured the mass difference between $^{72}$As and $^{72}$Ge, ruling out $^{72}$As as a candidate for neutrino mass determination via beta decay due to its negative $Q$-value.

Contribution

First direct measurement of the $^{72}$As to $^{72}$Ge ground-state $Q$-value with over 50-fold improved precision, clarifying its unsuitability for neutrino mass experiments.

Findings

$Q$-value measured to be 4343.596(75) keV

New $Q$-value is 12.4(40) keV lower than previous estimate

All potential ultra-low $Q$-value transitions are energetically forbidden

Abstract

We report the first direct determination of the ground-state to ground-state electron-capture $Q$-value for the $^{72}$As to $^{72}$Ge decay by measuring their atomic mass difference utilizing the double Penning trap mass spectrometer, JYFLTRAP. The $Q$-value was measured to be 4343.596(75)~keV, which is more than a 50-fold improvement in precision compared to the value in the most recent Atomic Mass Evaluation 2020. Furthermore, the new $Q$-value was found to be 12.4(40)~keV (3.1 $\sigma$) lower. With the significant reduction of the uncertainty of the ground-state to ground-state $Q$-value value combined with the level scheme of $^{72}$Ge from $\gamma$-ray spectroscopy, we confirm that the five potential ultra-low $Q$-value ${\beta^{+}}$-decay or electron capture transitions are energetically forbidden, thus precluding all the transitions as possible candidates for the electron neutrino mass determination. However, the discovery of small negative $Q$-values opens up the possibility to use $^{72}$As for the study of virtual $\beta$-$\gamma$ transitions.